1. Field of the Invention
The present invention pertains to a fracturing method for hydraulically fracturing an earth formation from a wellbore by over-pressuring the wellbore tubing or casing with artificial lift gas or gas from another source so as to provide a high pressure/flow condition during fracture initiation or extension.
2. Background
In hydraulically fracturing earth formations to stimulate production of fluids therefrom, a long-standing problem has been the inability to sustain high pressure and flow of the fracturing fluid during fracture initiation or extension. In deviated wells, in particular, inadequate pressure/flow conditions at fracture initiation will produce a near wellbore kink in the fracture which will tend to restrict the flow of fluids to or from the wellbore once the fracture has been formed. U.S. Pat. No. 5,074,359, issued Dec. 24, 1991 to Joseph H. Schmidt and assigned to the assignee of the present invention discusses the problem of improper fracture formation from deviated wellbores, in particular. This patent is directed to a method for orienting the casing perforations to minimize improperly formed fractures at or near the perforations.
Conventional hydraulic fracturing is limited by the inability to provide the fracture fluid at sufficiently high flow rate to sustain the formation breakdown pressure once breakdown occurs. Since, in conventional fracturing techniques, the fracture fluid is supplied at the requisite pressure from surface disposed pumps, pressure losses through the pumping system and the wellbore conduits leading to the fracture zone often preclude adequate fracture extension once breakdown occurs, and the resultant pressure drop in the flow path often provides improperly formed or inadequately extended fractures. Certain methods have been developed with a view to overcoming this long-standing problem. U.S. Pat. 4,633,951 to Hill, et al suggests filling the well casing with a compressible hydraulic fracturing fluid comprising a mixture of liquid, compressed gas and proppant material and then igniting a gas generating unit in the wellbore to generate a rapid outflow of gas followed by a charge of gassy fracturing fluid. However, the Hill et al method may not be suitable for fracturing wells which have been previously perforated nor wells which are operated on artificial gas lift. In any case, it has been considered desirable to provide a less complicated and less expensive fracture initiation and extension technique which is provided by the method of the present invention.